<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>BFC</title>
</head>
<body>

    <div>
        <div style="margin-bottom:200px">
            BFC作用域
        </div>
       <div style="margin-top:200px;overflow: auto;;">
        <div style="margin-top:200px;">
            dd
        </div>
        margin-top:200</div>
    </div>

</body>
</html>